Not Applicable
Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to a pressure sensor, and more particularly, a pressure sensor which relies on changes in capacitance to indicate pressure fluctuations.
2. Background of the Invention
Capacitive pressure sensors are well known in the prior art. Such sensors typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A displacable (relative to the fixed element) conductive member, such as a metal diaphragm, or a plated non-conductive member, such as a metalized ceramic diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. Because the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance C of the sensor may be approximated by the equation:                     C        =                              ε            ⁢                          xe2x80x83                        ⁢            A                    d                                    (        1        )            
where xcex5 is the permittivity of the material between the parallel plates, A is the surface area of the parallel plate and d represents the gap between the plates. The characteristic capacitance is inversely proportional to the gap between a central portion of the diaphragm and the conductive surface of the fixed element. In order to permit a pressure differential to develop across the diaphragm, the region on one side of the diaphragm is sealed from the region on the opposite side.
In practice, the diaphragm elasticity is selected so that pressure differentials across the diaphragm in a particular range of the interest cause displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates, and thus in capacitance variations produced by the sensor capacitor. For relatively high sensitivity, such sensors require large changes of capacitance in response to relatively small gap changes. Regarding equation (1), if xcex5 and A are held constant, the greatest slope of the d verses C plot occurs when d is small. Thus, for the greatest sensitivity, the gap is made as small as possible when the device is in equilibrium and the sensor is designed so that the gap d changes as pressure is applied. The multiplicative effect of xcex5 and A increases the sensitivity of the d to C relationship, so xcex5 and A are maximized to achieve the highest possible sensitivity.
In a typical prior art embodiment, the sensor capacitor formed by the fixed conductive surface and the diaphragm is electrically coupled via conductors to an oscillator circuit. The oscillator circuit typically includes an inductor that forms a tank circuit with the remotely located sensor capacitor. This LC tank circuit provides a frequency reference for the oscillator circuit; the output frequency of which is a direct function of the resonant frequency of the tank circuit. The resonant frequency of the tank circuit is in turn a direct function of the inductance L of the inductor and the capacitance C of the sensor capacitor. It is well known to those in the art that the resonant frequency xcfx890 of a simple LC tank circuit is given by             ω      0        =                  1                  LC                    .        ⁢      xe2x80x83  
As long as the values of the inductor and the capacitor both remain fixed, the output frequency of the oscillator circuit remains constant. However, since the capacitance of the sensor capacitor varies as a function of the pressure applied to the diaphragm, the output frequency of the of the oscillator circuit also varies as a direct function of the applied pressure.
Such a configuration produces a signal whose frequency is indicative of the pressure applied to the remote sensor. One disadvantage to this configuration is that having the capacitive sensor located remotely can introduce environmentally induced errors in the expected resonant frequency of the tank circuit. For example, it is well known to those in the art that the inductance value L of an inductor and the capacitance value C of a capacitor are each temperature dependent to some extent, depending upon the design of each particular physical component. The effect of the temperature on the capacitance or inductance of a particular component is often quantified as the xe2x80x9ctemperature coefficientxe2x80x9d associated with that component. It is possible to design a component so as to minimize the temperature coefficient, thus rendering the value of the device relatively insensitive to temperature, but commercially available components typically do have a measurable temperature coefficient which affects the component performance. It is also possible to choose components whose temperature coefficients are complementary, such that the net effect of a temperature change to the components together is nominally zero. However, when two components are not located together, such as the capacitive sensor and the inductor in the oscillator circuit, the ambient temperatures are often different, and complementary temperature coefficients do not produce a nominally zero sensitivity to temperature changes.
Another disadvantage to having a remotely located capacitive sensor is that the conductors used to electrically couple the sensor to the oscillator circuit introduce stray capacitances and inductances to the basic LC tank circuit. This disadvantage could be mitigated and thus acceptable if the stray values remained constant, but the stray values can change with environmental factors, physical movement of the conductors, etc.
It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.
The foregoing and other objects are achieved by the invention which in one aspect comprises a capacitive sensor for measuring a pressure applied to a conductive, elastic member, or a plated non-conductive elastic member, having at least a first substantially planar surface and being supported on at least one edge. The sensor includes a housing for supporting the elastic member by its edge, thereby forming (i) a controlled pressure chamber disposed on the side of the elastic member corresponding to the first planar surface, and a variable pressure region disposed on the side of the elastic member opposite said first side. The sensor also includes a capacitive plate disposed substantially adjacent to the elastic member so as to define a gap between the first planar surface and a corresponding planar surface of the capacitive plate. The gap, capacitive plate and elastic member together define a capacitor having a characteristic capacitance. The sensor further includes an elongated electrical conductor characterized by an associated inductance value. The conductor is fixedly attached to and electrically coupled with the capacitive plate. The gap between the capacitive plate and the elastic member varies as a predetermined function of the pressure applied to the elastic member so as to vary the characteristic capacitance. The capacitor and the electrical conductor together form a tank circuit having a characteristic resonant frequency; varying the capacitance of this tank circuit varies the resonant frequency of the tank circuit. Thus, the resonant frequency of the tank circuit is indicative of the pressure applied to the elastic member.
In another embodiment of the invention, the pressure applied to the elastic member is generated by a pressure differential across (i) the first planar surface of the elastic member and (ii) a second planar surface of the elastic member disposed substantially parallel to the planar surface. In one embodiment, this pressure differential is the result of a constant, controlled environment being in contact with the first planar surface, along with a fluid under pressure being in contact with the second planar surface of the elastic member.
In another embodiment, the electrical conductor is disposed in a spiral configuration within a plane substantially parallel to the capacitive plate.
In a further embodiment, the sensor further includes an insulator disposed between the capacitor plate and the electrical conductor. The insulator may be fixedly attached to either the capacitor plate, the electrical conductor, or both.
In another embodiment, the sensor further includes a stiffening element fixedly attached to the capacitive plate and the conductive element.